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    <script>
        // 引用类型错误 
        // 1. 引用未定义变量
        // Uncaught ReferenceError: num is not defined
        // console.log(a);



        // 2. 给一个方法赋值（Math）
        // Uncaught ReferenceError: Invalid left-hand side in assignment
        // let num = Math.abs(-100);
        // console.log(num);
        // Math.abs() = 'aaa';
    </script>
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